(a) Field of the Invention
The present invention relates to a video display. More specifically, the present invention relates to an organic electroluminescent (EL) display panel.
(b) Description of the Related Art
In general, an organic EL display electrically excites a phosphorous organic compound to emit light. A light emitting pixel includes an anode (ITO), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an EML (emitting layer), an ETL (electron transport layer), and an HTL (hole transport layer) for maintaining balance between electrons and holes and improving emitting efficiencies. The organic thin film further includes an EIL (electron injecting layer) and an HIL (hole injecting layer).
The organic emitting cells are arranged in an nxm matrix format to configure an organic EL display panel which displays video data by voltage or current-driving.
FIG. 1 shows a brief diagram of a general organic EL display.
As shown, the organic EL display includes an organic EL display panel 10, a scan driver 20, and a data driver 30.
The organic EL display panel 10 includes a plurality of data lines D1 to Dm extending in a column direction and arranged in a row direction, a plurality of scan lines S1 to Sn extending in the row direction and arranged in the column direction, and a plurality of pixel circuits 11. The data lines D1 to Dm transmit data voltages for displaying video signals to the pixel circuits 11, and the scan lines S1 to Sn transmit select voltages for selecting the pixel circuits 11 to the pixel circuits 11. The pixel circuits 11 are formed at pixel areas, each of which is defined by two neighboring data lines and two neighboring scan lines.
The scan driver 20 sequentially applies the select signals to the scan lines S1 to Sn, and the data driver 30 applies the data voltages for displaying the video signals to the data lines D1 to Dm.
Methods for driving the organic emitting cells through the pixel circuits 11 include a passive matrix method, and an active matrix method which uses thin-film transistors (TFTs). The passive matrix method forms anodes and cathodes to cross each other, selects lines, and drives the organic emitting cells, and the active matrix method uses the TFTs to select lines, stores data in capacitors of the pixels, and drives the organic emitting cells.
U.S. Patent Publication No. 2002/0118150 discloses a pixel circuit with four transistors for the organic EL display based on the active matrix method.
A driving transistor transmits the current which corresponds to a voltage between a gate and a source to an organic EL element (OLED). A capacitor is connected between the gate and the source of the driving transistor. A compensation transistor for compensating for a threshold voltage of the driving transistor is diode-connected, and its gate is coupled to the gate of the driving transistor. A switching transistor which has a gate coupled to a current scan line and a source coupled to a data line applies the data voltage to a source of the compensation transistor in response to the select signal of the current scan line. A pre-charge transistor which has a gate coupled to a previous scan line applies a pre-charge voltage to the gate of the driving transistor in response to the select signal of the previous scan line.
In order to make the threshold voltages at the driving transistor and the compensation transistor substantially the same in the above-described pixel circuit, the driving transistor and the compensation transistor are arranged in parallel with the data lines. The pre-charge transistor is arranged to be coupled to a previous scan line.
However, the above-mentioned arrangement has a problem of reducing the aperture ratio of display pixels since the area of gate lines for coupling a drain of the pre-charge transistor, the gate of the driving transistor, and the gate of the compensation transistor is increased.